Conventionally, an assembled battery in which battery stacks having a plurality of battery cells are connected to each other has been used as a power supply of an electric vehicle, a hybrid vehicle, etc. A device that monitors the assembled battery is connected to the assembled battery. The device detects the voltage of each battery cell of the assembled battery and discharges the battery cell in accordance with the detected voltage so as to adjust the capacity balance of the corresponding battery cell (see Japanese Laid-open Patent Publication No. 2014-033604).
However, the patent literature does not disclose that the device monitors an assembled battery and diagnoses a failure of battery cells.
A monitoring device detects the source voltage of a monitoring circuit or/and the voltage during the discharge and undischarge of battery cells in order to perform a failure diagnosis of a monitoring circuit and the battery cells. At this time, when detecting a voltage by using a free-running A/D converter that periodically operates regardless of the control of a monitoring device, for example, it is needed to perform a failure diagnosis in accordance with the operation period of the A/D converter, and thus a time needed for a diagnosis becomes longer. Moreover, when errors are included in the operation period of the A/D converter, for example, it is needed to perform a failure diagnosis in consideration of the impact of the errors. A time needed for a diagnosis becomes longer due to the impact of the errors.
As described above, because a time for adjusting capacity balance becomes shorter when a time needed for a failure diagnosis becomes longer, it is desired to reduce a diagnosis term of a failure.